Production of a microporous artificial leather coating



United States Patent 3,527,653 PRODUCTION OF A MICROPOROUS ARTIFICIALLEATHER COATING Erwin Summer, Obernburg, Klaus Gerlach, Obernau,

Werner Riess, Erlenbach, and Helmut Schaefer, Elsenfeld, Germany,assignors to Vereinigte Glanzstoif AG, Wuppertal, Germany No Drawing.Filed June 16, 1967, Ser. No. 648,186 Claims priority, application2Germany, June 18, 1966,

V 3 Int. Cl. D06n 3/14; B4411 1/44 US. Cl. 117--62.2 18 Claims ABSTRACTOF THE DISCLOSURE Process for making a microporous artificial leathercoating by depositing a polyurethane gel on a substrate, precipitatingthe gel at -80 C. with an aqueous electrolytic solution having apH-value of 3-9 and a concentration of the electrolyte of at least 15%by weight, and then washing and drying the gel to form a leather-likemicroporous structure. Metal salts such as sodium chloride or potassiumnitrate are useful as the electrolyte.

This invention is concerned with a process for producing leather-likelayers or coatings of polyurethane, preferably on a fibrous substrate soas to achieve an extremely fine pore structure as the outer surfacelayer of the finished article. This fine pore structure as a cover layermust be microporous in size and open-channeled to provide good air andvapor permeability while preventing the passage of liquid water.

It is known that leather-like fiat shaped structures can be producedfrom a textile substrate in which an elastomeric binding agent isuniformly distributed. In preparing the textile base layer or substrate,it is preferable to use non-woven fibrous sheets or mats as a felt-likematerial although it is also possible to use woven or knitted fibroustextiles or even several layers of woven and/or n0n-woven sheets mayalso be used. As the elastomeric binding agent, various polymers havebeen used, for example: polyurethane, polyvinyl chloride and butadiene/acrylonitrile copolymers. These polymers are generally sprayed onto thetextile substrate in the form of solutions or are applied by dipping,impregnating or doctoring onto the substrate.

Up to the present time, the best results have been achieved by treatingthe impregnated substrate with a non-solvent which can be at leastpartially mixed with the solvent which has been used for the appliedelastomeric binding agent. Then, after impregnating and precipitating orcoagulating the elastomer, both the solvent and non-solvent are washedout and the resulting structure is dried. While these textile baselayers having an elastomeric binding agent exhibit some leatherlikequalities or characteristics, it is still necessary to apply anadditional cover layer or coating onto the base layer in order toimprove the surface structure and appearance and also to obtain awatertight article, i.e. to prevent the passage of liquid water.

A number of processes have become known for obtaining such cover layersor coatings which are preferably applied to a leather-like,elastomer-bound textile substrate. In these processes, one generallyproceeds by first converting polyurethane or mixtures of polyurethanewith polyvinyl chloride into their gel form in a suitable solvent, andthen precipitating the gel on the textile base layer by treatment with anon-solvent, preferably water. In preparing the gel, the polymer isfirst dissolved in a solvent, e.g. dimethyl formamide being especiallyuseful as the solvent for polyurethane, and a non-solvent such as wateror alcohol is incorporated into this polymer solution. The quantity ofthe non-solvent is added or dosed in an amount which is just sufficientto gel the polymer. Such a gel can be considered as a two phase systemin which at least one substance is uniformly dispersed in anothersubstance, both the dispersed substance and the dispersing medium beingcontinuous or coherent phases which penetrate one another. (See, forexample, I. Stauff, Kolloidchemie, Springer-Verlag, 1961, p. 666.) Afterapplication of the polymer gel onto the textile layer or substrate,water or mixtures of water with dimethyl formamide are used as thecoagulating or precipitating bath. The polymer as the dispersed phasethereby hardens or coagulates and the dispersing medium as the secondphase can then be removed from the solidified structure by washing,thereby leaving an open-channeled or interconnected porous solidstructure.

In following such known procedures, however, it was learned that surfacestructures or layers are rarely produced so as to be completely free ofmacropores. Another disadvantage of known methods for producing suchlayers resides in the fact that the dosage rate at which nonsolvent andsolvent are added is quite critical and must be precisely controlled.Relatively small deviations in the amount or rate of this dosage duringthe process results in a gel structure having very undesirablecharacteristics, either in its pore structure or a non-homogeneous gel.

The macropores are defined as pores in the hardened or solidified gelwhich are so large in size as to be visible with the naked eye down tosmaller sizes which are still visible or recognizable as small vacuoleson a photographic cross-section of the porous layer enlarged on a scaleof approximately 1:100. These macropores generally lie close to ordirectly at the surface of the cover layer and are covered by anextremely thin film of the solid polymer or are separated from oneanother by such a thin film or membrane. This not only has a veryunfavorable effect on the air permeability and water conductivity of theleather-like surface coating but also has a detrimental effect on itselastic qualities.

By contrast, micropores are defined as those pores in the leather-likecoating or surface structure which are sufiiciently small in size sothat no vacuoles can be seen in a cross-sectional photographicenlargement of 1:100. Thus, in a perfectly microporous surface layer, acrosssectional enlargement of times actual size will not show anyevidence of vacuoles or bubbles which are usually covered or separatedby a thin film of polymer.

One object of the present invention is to provide a process for making amicroporous leather-like polyurethane coating or layer which issubstantially free of macropores and exhibits improved elasticproperties. Another object of the invention is to provide a process bywhich the desired microporous structure can be formed under conditionswhich are more easily controlled. These and other objects and advantagesare more fully explained in the following detailed description.

It has now been found, in accordance with the present invention, thatone can achieve a leather-like microporous coating, film or sheetmaterial which is substantially free of macropores if a polyurethane gelinitially applied to a substrate is coagulated or hardened by treatmentat about to 80 C. with an aqueous electrolytic solution having apH-value between 3 and 9, preferably about 4.5 to 8, and a concentrationof the electrolyte of at least by weight, preferably more than byweight, up to the saturation point of the electrolyte in the aqueoussolution, i.e. up to the point at which the electrolyte forms asaturated solution in water at the coagulation temperature. Thereafter,the coagulated gel is washed and dried in conventional manner to removethe liquid portion thereof and the electrolyte, i.e. the soluble saltcontent of the gel.

The polyurethane gel can be applied to many different substratesdepending upon the particular use of the final leatherlike film, sheetor cover layer, and in most instances is applied onto a fibroussubstrate, e.g. woven and non-woven fiber-containing fabrics, fleeces orsimilar textile materials which may or may not be impregnated with asuitable binding agent. Paper sheets composed of natural, artificialand/or synthetic organic fibers may also be used as a substrate, or onecan even produce a microporous sheet or film by using a substrate fromwhich the coagulated gel can be easily removed by stripping or peelingafter being treated in accordance with the process of this invention.The invention is especially applicable, however, in providing animproved microporous surface coating on a fibrous non-woven fleece whichhas been impregnated with an elastomeric polymer to provide aleather-like, fiber-reinforced base layer. The gel can be prepared andapplied according to conventional processes, e.g. by spreading,brushing, extrusion, coating with a doctor blade or the like.

Polyurethanes which are suitable as a film-forming or coating materialon a flat substrate to provide a porous elastomeric polymer surfacelayer are generally quite well known in this art. For purposes of thepresent invention, the desired elastomeric layer should consistpredominately of a suitable polyurethane but may also contain a minorproportion of one or more other film-forming thermoplastic organicpolymers of the many polymers and copolymers which are known to becompatible with a polyurethane gel, preferably other elastomericpolymers. For example, one can use a mixture of polymers in which thepolyurethane content is at least 50% by weight and preferably about 65%by weight or more.

Generally speaking, polyurethanes are produced by reacting adiisocyanate with one or more organic compounds or substances whichcontain terminal hydroxy groups or reactive hydrogen atoms, especiallywith such polymers as polyethers, polyesters or polyesteramides, oftenwith the further interaction of diamines, water or glycols. For purposesof the present invention, the polyurethane must be soluble in organicsolvents suitable for gel formation and also should be capable of beingcoagulated or solidified from the gel so as to provide a porousstructure which is insoluble in water. Suitable polyurethanes of thistype, including compositions containing minor amounts of otherfilm-forming thermoplastic polymers are disclosed in such references asU.S. Pats. No. 2,871,218 and No. 3,190,766.

For example, in preparing the polyurethane, it is quite common to usesuch diisocyanates as tolylene-2,4- or -2,6-diisocyanate,diphenylmethane-4,4-diisocyanate, hexamethylene diisocyanate,naphthalene diisocyanates or various other alkylene and/or arylenediisocyanates. Examples of preferred types of linear polymers to bereacted with the diisocyanate in forming the polyurethane include thefollowing: polyethers such as polyethylene ether glycols, polypropyleneether glycols, polyhexamethylene ether glycols and the like; polyesterssuch as tetramethylene glycol adipate, hexamethylene glycol adipate,hexamethylene glycol terephthalate and the like; and polyesteramideswhich are the amides of the previously noted polyesters. There may alsobe reacted with the diisocyanate and the linear polymer any number ofother compounds having reactive hydrogen atoms including: glycols suchas ethylene glycol, tetramethylene glycol, hexamethylene glycol;diamines such as ethylene diamine, phenylene diamine and dimethylpiperazine; or even small amounts of diethylene diimine.

In most cases, it is essential to avoid extensive crosslinking in theinitial polyurethane to be formed into a gel so that substantiallylinear polyurethanes are especially desirable, and in general suchpreferred polyurethanes should have a relative viscosity of about 20 to100 poise as measured in a 20% dimethyl formamide solution at 20 C.(i.e. 20% by weight of the polyurethane being dissolved in the solvent).

It will be apparent from the foregoing description of polyurethanes ortheir combination with other film-forming or elastomeric polymers thatwhen referring to a polyurethane gel, a polyurethane polymer or apolyurethane elastomer, the present invention is concerned with arelatively broad and well-known class of polymers or polymer mixturesused in the preparation of porous surface structures from a gel. In manycases, however, the amount of polymers other than the polyurethanepolymer does not exceed 5 to 10% by weight and for practical purposesmay be considered as being essentially a linear polyurethanecomposition. This is also true with respect to the addition of smallamounts of so-called chain extenders or even small amounts of across-linking agent, it being understood that those skilled in this artcan readily prepare substantially linear polyurethanes and their gelswithin the scope of such minor modifications.

Good results have been achieved by preparing a polyurethane gel whichcontains about 15-35% by weight of the linear polyurethane, 0 to 15 byweight of one or more of the other film-forming thermoplastic polymers,5 to 30% by weight of a non-solvent for the polyurethane and 20 to byweight of a solvent for the polyurethane in which the non-solvent is atleast partly soluble or miscible. Especially preferred proportions ofthe gel components are as follows:

Percent by weight Particularly suitable polymers other thanpolyurethanes contained in the gel composition include, for example:polyvinyl chloride; various copolymers of vinyl chloride, especiallyvinyl chloride/maleic acid anhydride or vinyl chloride/vinyl acetatecopolymers; and butadiene-acrylonitrile and/or -styrene interpolymers.Chlorinated rubber or nitrocellulose are likewise especially useful. Thenitrocellulose preferably has a nitrogen content of about 10.8 to 14% byweight. As additional examples of a number of suitable copolymers whichcan be at least partially mixed into the gel-forming compositioncontaining polyurethane, the following table has been compiled as afurther illustration of the additional film-forming thermoplasticpolymers. All percentages are by weight with reference to the totalweight of the copolymer so as to indicate the approximate proportions ofthe monomers.

TABLE I (A) Vinylchloride copolymers (l) 80% vinyl chlorides-10% ethylmaleate, 10% methyl maleate;

(2) 80% vinyl chloride+20% methyl acrylate;

(3) 80% vinyl chloride+10% methyl acrylate, 10% isobutyl maleate;

(4) 87% vinyl chloride+13% vinyl acetate;

(5) 80% vinyl chloride+20% ethyl maleate;

(6) vinyl chloride+l5% vinylidene chloride;

(7) 80% vinyl chloride+20% maleic acid anhydride;

(8) 70% vinyl ch1oride+30% vinylisobutylether.

(B) Butadiene/acrylontrile copolymers (9) 72% butadiene+28%acrylonitrile; (10) 10% butadiene+90% acrylonitrile;

(and all proportions of butadiene and acrylonitrile falling within theselimits).

C) Styrene/butadiene copolymers (11) 80% styrene+20% butadiene.

(D) Vinylidene chloride/acrylonitrile/methacrylate copolymers (12) 80%vinylidene chloride+10% acrylonitrile, 10%

methyl methacrylate.

Regardless of whether the gel-forming composition consists essentiallyof a polyurethane or contains admixed therewith a relatively substantialamount of another filmforming, water-insoluble polymer, the procedurefor producing the gel is generally well known. The polymer or polymermixture is dissolved in a suitable solvent, preferably dimethylformamide although dimethyl acetamide is also quite useful. Other Wellknown solvents for polyurethanes include dimethyl sulfoxide,tetrahydrofuran, gamma-butyrolactam, gamma-butyrolactone andepsiloncaprolactam. In general, the polyurethane gel utilized in thepresent invention is most conveniently prepared with dimethyl formamideas the solvent.

In order to precipitate the gel from the polymer solution (which mayalso be considered a colloidal solution or suspension of the polymer inthe solvent), it is necessary to add a suitable non-solvent up to thepoint where gel formation takes place or just slightly beyond thispoint. Suitable non-solvents for the preparation of polyurethane gelsinclude water or lower alkanols such as methanol and ethanol. While thenon-solvent is usually a liquid in which the polymethane elastomer isinsoluble, it is also feasible to use ammonium nitrate as aprecipitating agent in the form of a solid non-solvent or else ammoniumnitrate dissolved in dimethyl formamide or other suitable sol vent forthe polyurethane can be used as a liquid nonsolvent medium. In eithercase, the non-solvent should be at least partially soluble or misciblein the dimethyl formamide or other suitable solvent. The gels preparedfor use in this invention contain as the preferred nonsolvent about5-10% water or 5-30% by weight of ammonium nitrate.

In applying the gel to the substrate, e.g. a fibrous fleece or non-woventextile base layer, regardless of the particular technique used such asspraying, dipping or doctoring, it is usually most desirable to providea layer thickness of about 0.4 to 1.5 mm. where the end-product is tohave the surface appearance and characteristics which simulate naturalleather. Of course, the applied gel may also contain pigments or dyes,and one can also add inert thickening agents, fillers or extenders in aconventional manner.

The novel and critical step essential for achieving the improved resultsof the present invention is the coagulation or hardening of the appliedpolyurethane gel by treatment with the prescribed electrolytic solution.The following details of this step are therefore quite important andshould be carefully observed in order to obtain the desired product.

The treatment with the aqueous electrolytic solution takes place in theusual form of a liquid bath although it is also feasible to spray orflow the treating solution onto the gel-coating provided that thetemperature is maintained within the range of approximately 5 to 80 C.,preferably 15-60 C.

The electrolytic solution is essentially an aqueous solution of awater-soluble metal salt in a concentration of at least 15 by weight andgenerally more than 20% by weight. The upper limit of concentration isnot critical since saturated solutions of the metal salt at thecoagulation temperature, i.e. between 5 and 80 C. are quite suitable.Both inorganic and organic metal salts, i.e. the metal salts ofinorganic and organic acids are useful provided that the resultingsolution in the prescribed concentration yields a pH-value of 3-9 andpreferably from about 4.5 to 8. Mixtures of salts can also be used asthe electrolyte provided that the mixture is soluble in water in therequired concentration. Furthermore, it has been found that it is quiteadvantageous to add a small amount of dimethyl formamide to theelectrolytic solution, e.g. up to 15% by weight and preferably about510% by weight of dimethyl formamide with reference to the total weightof the electrolytic bath.

The aqueous electrolytic solution can be prepared by dissolving in waterany metal salt which will yield the desired pH and concentrationrequired for the treatment bath. For example, one can select any numberof suitable salts of the metals of Groups I and II of the PeriodicTable, preferably the alkali metal and alkaline earth metal salts.Ammonium and zinc salts are also quite useful for purposes of thepresent invention. There can be used salts of. mineral acids such as thephosphates, sulfates, chlorides and nitrates, or the salts of organicacids such as the acetates and formates. From readily available data orby a simple preliminary test, it can be determined whether anyparticular salt will meet the pH and concentration limitations of themethod of the invention.

For convenience, the following table lists a number of salts which canbe used as the electrolyte according to the invention together withtheir solubility in water at 20 C. This table is intended as beingillustrative only.

TABLE II Solubility in H O at 20 C. (percent) 26.4 46.8 27.1 65.2 25.662.9 43.0

The precipitation, coagulation or hardening of the elastomer gel by theelectrolytic solution requires the bath to be in intimate contact withthe gel under controlled temperature conditions, but the time requiredto complete the coagulation is not critical. In general, the duration ofthe bath treatment need not exceed 15 minutes and shorter periods oftime are usually quite suitable. The reaction time required forcoagulation naturally depends upon the temperature of the coagulatingboth. In the case of very hot baths of about C., the precipitation orcoagulation of the gel can be substantially com pleted after only aboutone minute. In order to assure a uniform microporous structure of thecoating or surface layer, it has been found advantageous to complete theprecipitation of the gel with the electrolytic solution within a periodof about 2 to 10 minutes.

After the polymer gel has been coagulated, it is washed with water in aconventional manner so as to substantially completely remove the solventand non-solvent components of the original gel and especially to removethe water-soluble salts. After washing, the coagulated gel is dried inthe usual way, e.g. by using heated air. The use of salt solutions as anelectrolytic bath does not prolong the time normally required forwashing the coagulated gel since the water-soluble salts used as theelectrolyte are more easily extracted than the solvents such as dimethylformamide.

The following examples will serve to further illustrat the invention:

7 EXAMPLE 1 According to known procedures, there was first produced afibrous fleece by carding polyamide (nylon) and regenerated cellulose(rayon) fibers of a conventional staple length, and this carded fleecewas impregnated with a linear polyurethane corresponding to that to beapplied subsequently as a surface coating. After hardening thepolyurethane to form an elastomeric binder for the fleece and washingand drying the resulting fibrous sheet, a polymer gel was applied bymeans of a doctor blade to a thickness of 1.1 mm. The polymer gelconsisted of 20% by weight of a linear polyurethane, 4% by weight ofpolyvinyl chloride, approximately 6% by weight of ammonium nitrate andapproximately 70% by weight of dimethyl formamide.

The textile base layer or substrate coated with this gel was thenconducted through an aqueous coagulating bath consisting of an aqueoussolution of 20% by weight NaCl at a bath temperature of 20 C. After areaction period or time of treatment of five minutes, the coagulated gelwas washed with water and dried.

The resulting product exhibits a surface layer having a highly uniformmicroporous structure. Pores with a diameter of 10 microns or more couldnot be observed, most of the micropores being much smaller. The finishedarticle exhibited a permeability to water vapor of 650 g./m. /day. In acomparative test wherein the same procedures were followed except thatpure water was used in coagulating the applied gel, the finished articlehad a surface coating in which there were dispersed a large number ofmacropores with a diameter above 20 microns.

EXAMPLE 2 As described in Example 1, the fiber fleece sheet material wascoated with a surface layer of the polymer gel. However, in order tocoagulate the gel, an electrolytic bath was used which contained 10% byweight dimethyl formamide as Well as the 20% by weight NaCl. Thetreatment with this bath took place at a temperature of 50 C. Theresulting leather-like product has an extremely fine pored surfacestructure. In testing for water vapor permeability according to DIN53333 (German Industrial Standard), the finished article exhibited avalue of 750 g./m. day.

EXAMPLE 3 The same procedure was followed as in Example 1 except thatthe coagulating bath consisted of a 50% by weight solution of MgCl inplace of the sodium chloride solution. The polyurethane surface coatingwas again found to be substantially free of macropores while exhibitinga uniform distribution of micropores.

EXAMPLE 4 By following exactly the same procedure as in Example 1 butusing a 50% by weight sodium acetate solution in water as the aqueouscoagulating bath to precipitate the gel coating, it was again possibleto obtain a uniform microporous surface structure substantially free ofmacropores.

EXAMPLE 5 A non-woven fibrous fleece in the form of a sheet was producedin the usual manner on a papermaking machine from staple lengths ofpolyamide (nylon) and regenerated cellulose (rayon) fibers. The fibroussheet was then impregnated with an elastomeric binder consisting of alinear polyurethane in admixture with polyvinyl chloride, and thisbinder was then hardened, washed with water and dried to form a textilebase layer. Onto this base layer there was applied a surface coating orlayer of 0.8 mm. thickness of a polymer gel having approximately thefollowing composition: 25% by weight of polyurethane; 5% by weight of acopolymer of 80 parts by weight of vinyl chloride to 20 parts by weightof maleic 8 acid anhydride; 10% by weight of water and 60% by weight ofdimethyl formamide.

The coating substrate was then conducted through a coagulating bath ofwater containing 32% by weight of KNO as the electrolyte whilemaintaining the bath temperature at C. The coated substrate had aresidence time of 3 minutes in this bath. Subsequently, the coagu latedcoating was washed with water and dried to yield a very fine and uniformporous surface structure substantially free of macropores.

EXAMPLE 6 The procedure of Example 5 was repeated except that thecoagulating bath consisted of an aqueous 19% by weight solution of KNOWith this lower concentration of the electrolyte, a few macroporesappeared in the otherwise microporous coating.

EXAMPLE 7 As described in Example 5, the polymer gel was appiled to thenon-woven fibrous substrate impregnated with the elastomeric binder. Thepolymer gel had the same composition as described in Example 5 exceptthat the vinyl chloride/maleic acid anhydride copolymer was rereplacedby nitrocellulose in the same quantity, i.e. 5% by weight with referenceto the total weight of the gel.

In order to coagulate or precipitate the polymer gel as a surfacecoating, the coated substrate was conducted through a 50% by weight NHNO solution in water, the bath temperature being maintained at 23 C.,for a total residence time in the bath of 8 minutes. After washing withwater and drying, a fiat surface coated microporous structure resemblingleather was obtained. This finished product remained undamaged after500,000 flexures during a test of its flexing or bending strengthcarried out in accordance with 1UP 20 by means of a Bally flexometer.

EXAMPLE 8 The procedure of Example 7 was repeated except that theconcentration of the NH 'NO in the coagulating bath was varied in aseries of tests in which this concentration was reduced from 50% to 10%.At a concentration of 25% by weight of NH NO some macropores can beobserved which increase in size and number as the concentration of thesalt in the coagulating bath decreases. During these tests the bath hada pH-value of between about 5 and 6.

EXAMPLE 9 Using a polyurethane impregnated fibrous fleece for thetextile base layer as in the preceding examples, a polymer gel coatingwas applied on the surface thereof to a thickness of 0.5 mm. Thispolymer gel contained approximately the following amounts of individualcomponents: 24% by weight of a linear polyurethane; 6% by weight ofammonium nitrate and by weight of dimethyl formamide. In order tocoagulate the gel coat-, ing, it was conducted on the substrate throughan aqueous electrolytic solution consisting of 22% by weight of amixture of 70 parts by weight NaCl to 30 parts by weight KCl. Thetemperature of the solution was maintained at 30 C. during the bathtreatment. After Washing with water and drying, a very fine-poredsurface structure was obtained. By photographing a cross-section of thissurface coating and enlarging in a ratio of 1:100, no macropores couldbe observed and the product exhibited a highly uniform microporousstructure.

In each of the foregoing examples, the linear polyurethane used to formthe surface layer or coating on the textile substrate was prepared asdescribed in US. 2,871,- 218 from polybutyladipate (molecular weight of1000), diphenyl-methane-4,4-diisocyanate and a small amount ofbutanediol as a chain extender. This is the polyester type ofpolyurethane and has a solution viscosity of 50-70 poise when measuredas a 20% solution in dimethyl formamide at 20 C. This same linearpolyurethane can be used to impregnate the fibrous substrate as anelastomeric binding agent, preferably in admixture with polyvinylchloride as pointed out in Examples 1 and 5. However, the particularsubstrate is not critical, the examples herein using an impregnatedfibrous fleece as an illustration of a preferred artificial leatherproduct.

The aqueous electrolytic solution employed as the coagulating bath ineach of the foregoing examples exhibited a pH-value ranging betweenabout 5 and 9 depending upon the particular electrolyte used in eachcase. One should avoid using salts which result in an excessively lowpH-value, e.g. iron chloride or aluminum chloride, because the polymercoating then tends to become shriveled or exhibit a wrinkled or puckeredsurface. The minimum concentration of the electrolyte will tend to varyto some extent, but it has been found that more than 20% by weight isusually required, and the formation of macropores can be eliminated inany case by simply increasing the concentration up to the saturationvalue of the electrolyte in Water at the coagulation temperature.

In addition to an excellent microporous structure, the coating orcovering layer produced according to the invention has a distinctlybetter resistance to bending, i.e. a longer flexing life, than coatingswhich still contain macropores. Where the polymer surface structurecontains such macropores, the thin film of polymer above or between thevisible bubbles or vacuoles break or tear during a repeated flexuralstrength test. Once this break takes place, the crack in the surfacecoating continues to tear very quickly and extends through the rest ofthe structure. In comparative tests according to IUP 20 on a Ballyflexometer (Messrs. Bally A. G., Schoenenwerd, Switzerland), coverlayers or coatings containing such macropores are destroyed, on anaverage, after about 10,000 fiexures, while the microporous coatingsproduced according to the present invention remain undamaged up to about200,000 fiexures or more.

The polyurethane polymer coatings or fiat surface film structures of thepresent invention are thus distinguished not only by their excellentmicroporous structure but also by their resistance to bending orflexing. Since artificial leather products are often subjected to suchstress, for example when used in footwear or the like, an increasedflexing life is a very substantial advantage. At the same time, auniform microporous structure contributes to the desired air and watervapor permeability of the artificial leather product.

The invention is hereby claimed as follows:

1. A process for the production of a microporous artificial leathercoating which comprises: applying to a porous substrate a polyurethanegel containing a solvent for said polyurethane, coagulating the appliedgel at a temperature of about 5 to 80 C. with an aqueous electrolyticsolution having a pH-value between 3 and 9 and a concentration of awater soluble metal or ammonium salt electrolyte of at least by Weightup to the saturation point of the electrolyte in said aqueous solution,and subsequently washing and drying the coagulated gel to form amicroporous coating.

2. A process as claimed in claim 1 wherein the polymer of said gelconsists essentially of a linear polyurethane having a relativeviscosity of to 100 poise measured in a 20% dimethyl formarnide solutionat 20 C.

3. A process as claimed in claim 1 wherein said gel contains 15 to 35 byweight of a linear polyurethane, up to 15% by weight of at least oneother film-forming thermoplastic polymer, 5 to 30% by weight of anonsolvent for polyurethanes and 20 to 80% by weight of a solvent forpolyurethanes in which said non-solvent is as least partially soluble.

4. A process as claimed in claim 1 wherein said gel contains 20 to 30%by weight of a linear polyurethane, up to 10% by weight of at least oneother film-forming thermoplastic polymer, 5 to 15% by weight of anonsolvent for polyurethanes and 45 to 75% by weight of a solvent forpolyurethanes in which said non-solvent is at least partially soluble.

5. A process as claimed in claim 3 wherein the polymer content of saidgel consists essentially of a linear polyurethane having a relativeviscosity of 20 to 100 poise measured in a 20% dimethyl formamidesolution at 20 C. and at least one other polymer selected from the classconsisting of polyvinyl chloride, vinyl chloride/ maleic acid anhydridecopolymers, vinyl chloride/vinyl acetate copolymers, and copolymers ofbutadiene with at least one of the monomers acrylonitrile and styrene.

6. A process as claimed in claim 3 wherein the polymer content of saidgel consists essentially of a linear polyurethane having a relativeviscosity of 20 to 100 poise measured in a 20% dimethyl formamidesolution at 20 C. and nitrocellulose.

7. A process as claimed in claim 3 wherein Water is used as thenon-solvent in an amount of about 5 to 10% by weight with reference tothe total Weight of the gel.

8. A process as claimed in claim 3 wherein ammonium nitrate is used asthe non-solvent in an amount of 5 to 30% by weight with reference to thetotal weight of the gel.

9. A process as claimed in claim 3 wherein said solvent is dimethylformamide.

10. A process as claimed in claim 1 wherein said coagulation of theapplied gel is carried out at a temperature of about 15 to 60 C. with anaqueous electrolytic solution having a pH-value of about 4.5 to 8 and aconcentration of the electrolyte of at least about 20% by Weight up tothe saturation point of the electrolyte in said aqueous soltuion at thecoagulation temperature.

11. A process as claimed in claim 10 wherein said electrolytic solutioncontains approximately 5 to 10% by weight of dimethyl formamide withreference to the total weight of the solution.

12. A process for the production of a microporous artificial leatherfilm layer which comprises: applying to a porous substrate apolyurethane gel containing about 15 to 35% by weight of a linearpolyurethane having a relative viscosity of about 20 to 100 poisemeasured in a 20% dimethyl formamide solution at 20 C., from 0 up to 15%by weight of another film forming thermoplastic polymer, about 5 to 30%by Weight of a nonsolvent for polyurethanes and about 20 to by weight ofa solvent for polyurethanes in which said non-solvent is at least partlysoluble; coagulating the applied gel at a temperature of about 5 to 80C. by treatment thereof with an aqueous solution of at least about 25%by weight of a Water soluble electrolyte selected from the classconsisting of the salts of alkali metals, alkaline earth metals, zincand ammonium, said electrolyte being sufiicient to provide anelectrolytic solution with a pH-value of about 3 to 9; and subsequentlywashing the coagulated gel to form a microporous layer substantiallyfree of macropores.

13. A process as claimed in claim 12 wherein said solvent forpolyurethanes is dimethyl formamide.

14. A process as claimed in claim 12 wherein said electrolyte has ananion selected from the class consisting of sulfate, phosphate,chloride, nitrate, formate and acetate.

15. A process as claimed in claim 12 wherein said electrolyte is analkali metal salt.

16. A process as claimed in claim 12 wherein said other thermoplasticpolymer is selected from the class consisting of polyvinyl chloride,vinyl chloride copolymers, and copolymers of butadiene with at least oneof the monomers acrylonitrile and styrene.

17. A process as claimed in claim 12 wherein said other thermoplasticpolymer is nitrocellulose.

18. A process as claimed in claim 12 wherein said 3,208,875 9/1965Holden 117-63 X coagulating treatment is carried out for a period ofabout 3,214,290 10/1965 Larner et a1 117135.5 1 to 15 minutes. 3,238,0553/1966 Brightwell 117135.5 X

References Cited 3,348,963 10/ 1967 Fukushima et a1. 11763 5/1323 g gWILLIAM D. MARTIN, Primary Examiner 219501214 8/1960 Smith 117-612LUSIGNAN Assistant Examiner 3,100,721 8/1963 Holden 117-135.5 3,190,7656/1965 Yuan 117-63 CL 3,190,766 6/1965 Yuan 117--63 117-63, 135.5, 161

